Scientists have long sought to invent materials that can respond to the external world in predictable, self-regulating ways, and now a new study led by researchers at the University of Massachusetts (UMass) Amherst brings us one step closer to that goal. For their inspiration, the researchers looked to nature.
Lampreys swimming, horses walking and insects flying: each of these behaviors is made possible by a network of oscillators – mechanisms that produce a repetitive motion, such as wriggling a tail, taking a stride or flapping a wing. What's more, these natural oscillators can respond to their environment in predictable ways. In response to different signals, they can rapidly change speed, switch between different modes or stop changing altogether.
"The question," says Hyunki Kim, a PhD student in the Department of Polymer Science and Engineering at UMass Amherst, "is can we make soft materials, such as plastics, polymers and nanocomposite structures, that can respond in the same way?" The answer, as Kim and his colleagues report in a paper in the Proceedings of the National Academy of Sciences, is a definitive yes.
One of the key difficulties the researchers solved was getting a series of oscillators to work in unison with each other, a prerequisite for coordinated, predictable movement. "We have developed a new platform where we can control with remarkable precision the coupling of oscillators," says Ryan Hayward, professor of chemical and biological engineering at the University of Colorado Boulder, and one of the paper's co-authors.
This platform relies on yet another natural force, known as the Marangoni effect, which is a phenomenon that describes the movement of solids along the interface between two fluids driven by changes in surface tension. A classic, real-world example of the Marangoni effect happens every time you wash the dishes.
When you squirt dish soap into a pan filled with water on whose surface is evenly sprinkled the crumbs from your dinner, you can watch as the crumbs flee to the edges of the pan once the soap hits the water. This is because the soap changes the surface tension of the water, and the crumbs are pulled away from areas of low, soapy surface tension towards the edges of the pan where the surface tension remains high.
"It all comes down to understanding the role of interfaces and the profound impact of combining polymeric and metallic materials into composite structures," says Todd Emrick, professor in polymer science and engineering at UMass Amherst and another of the paper's co-authors.
Instead of soapy water and pans, the team used hydrogel nanocomposite disks made up of polymer gels and nanoparticles of gold, which were sensitive to changes in light and temperature. The result was that the team was able to engineer a diverse array of oscillators that could move in unison with each other and respond predictably to changes in light and temperature. "We can now engineer complex coupled behavior that responds to external stimuli," says Kim.